The transforming gene of Harvey murine sarcoma virus has apparently evolved from either of two structurally distinct cellular genes represented in a rat genomic library. The relative ability of these genes to direct the synthesis genomic library. The relative ability of these genes to direct the synthesis of the transforming gene product was tested by placing them under the control of SV40 late regulatory elements. One of these genes (which contains several introns) produces both messenger RNA and the transforming protein; the other gene, which lacks introns, is transcribed efficiently but is translated in greatly reduced amounts. Since the "intron-less" gene cannot transform NIH 3T3 cells under the control of a retroviral long terminal repeat, the above experiments suggest that a mutation which reduces translational efficiency resides either within or very near the coding region of this gene.